1. Field of the Invention
The present invention relates to a B picture mode determining method and apparatus in a video coding system, and more particularly, to a B picture mode determining method and apparatus in a video coding system in which direct prediction modes are selected as many as possible in a coding system using a B picture mode to thereby reduce a bit rate of the coding system.
2. Description of the Background Art
A ‘B’ picture in a video coding system such as an H.26L can be predicted from either or both of a previous and subsequent decoded ‘P’
Thus, a coding system using the ‘B’ picture can obtain an improved coding efficiency compared to a coding system that uses only the ‘P’ picture, and especially, it can obtain a considerable gain for low motion image sequences.
The B pictures in the video coding system are coded in five modes such as an intra prediction mode having 4×4 and 16×16 block size, a forward prediction mode, a backward prediction mode, a bi-directional prediction mode and a direct prediction mode.
Among the modes, the optimum mode determining method of a macro block (a basic unit of an image to be coded) as shown in FIG. 1 is that a sum of absolute difference (SAD) value, a prediction error measuring method between an original image and a predicted image, is calculated for the prediction modes, and one of them, which has the minimum SAD value, is selected as a prediction mode for the macro block.
In such a mode determining method, even though the optimum mode is determined on the basis of the SAD value according to the prediction error, there is always an overhead bit that is to be necessarily added according to a syntax of the mode.
The overhead bit affects the length of the overall bit stream, and accordingly, as bit rate is increased, degradation of coding performance of the video coding system may occur.
For example, if the bi-directional prediction mode is determined to be an optimum mode having the least SAD value, an overhead bit of four components such as reference frame information, forward and backward block size information, forward motion vector information and a backward motion vector information should be added to the bit stream to be coded, so that as the bit rate is increased, the coding performance of the video coding system is degraded.